As a heads up for audio:
The best quality that you could get is any lossless codec (flac, wav, aiff, etc.) but considering the size these files take up (10MB/min), I highly doubt you want to make your final video with them if size matters. This is especially true when people with high end setup's (e.g. $2000+ invested in their audio system) have a really hard time distinguishing these files from high quality lossy sources (mp3,aac,etc).
Therefore, don't even bother packaging lossless audio, stay with lossy. How compressed you want to get is another question. As a starting point, most people with these high-end setups have a hard time distinguishing 256kbps mp3 from 320kbps mp3 music, so some space can be saved even if you want maximum quality. 256kbps audio is probably the best quality/size ratio audio that you can get, so if you don't know what to go with use this.
The next step down in quality is 192kbps, and here is where most people can start telling the difference between max quality audio. However, the imperfections at this level are relatively minor and are difficult to spot if you don't have the reference track on hand. Because of this, 192kbps audio is a great choice if you are concerned about space but don't want people with high-end setup-ups to stop watching because of awful quality.
Below 192kbps, flaws start to become more obvious, but even still most people have problems distinguishing 128kbps from 192kbps in a blind situation (once again with high-end setups). If you are really cramped for space, go with 128kbps audio.
However, the individual song in question affects how kind the compression algorithms are. Some songs may sound remarkably different when comparing the 256+,192,and 128 versions, and even more bizarre some songs may sound better at 128 than lossless!
In the end, take into account your space restrictions and use your ear. Sometimes you might be able to drop your audio quality and sometimes you might need to raise it to achieve the same quality level that you want.
Also, as an extra tip, I highly recommend working with the uncompressed version (or at least the highest quality compressed version that you can find) and then down converting to your desired quality. Converting an already compressed file to an even more compressed format is going to lose a bunch of information that you really didn't need to lose.
Finally, Youtube quality standards are such that 720p+ video has at most 192kbps aac audio. Therefore, unless you plan on releasing a non-youtube copy (or if you want a super high quality version for yourself) there is no need to go higher than 192kbps if space is a massive concern.